The present invention generally relates to surface acoustic wave (SAW) duplexers, and more particularly to, SAW duplexers used it portable terminals, such as mobile telephones.
Recently, there is a demand to manufacture portable terminals that are smaller in size and lighter in weight, especially for mobile phones. For example, SAW devices have been employed in portable terminals. Such SAW duplexers have come into practical use for 800 MHz band of mobile communication, and are desired to be used for 2 GHz band communication too. It is required that a SAW duplexer has a high level attenuation in an attenuation band.
A conventional SAW duplexer includes a transmitting filter, a receiving filter and a branching circuit (branching line). The transmitting filter and receiving filter are applied with 1 to 2 watt and 1 watt of electric power, respectively. Therefore, the transmitting filter and receiving filter are required to have a power-resistance characteristic. The transmitting filter is composed of four-section T-type filter, having polarization characteristic. The receiving filter is composed of six-section xcfx80 ladder-type filter, a polarization characteristic. According to the above-described conventional SAW duplexer, the receiving filter is influenced by impedance of the transmitting filter especially in a transmitting band of 1920-1980 MHz and a receiving band of 2110-2170 MHz. As a result, it is difficult to make the receiving filter so as to have an insertion loss within a predetermined allowable standard range.
Further, according to the conventional SAW duplexer, the transmitting filter and receiving filter are mounted in a multi-layered substrate package. As a result, it is almost impossible after fabrication to adjust or control characteristics of the transmitting filter and receiving filter, including insertion loss and coupling between adjacent transmitting lines.
Accordingly, it is an object of the present invention to provide a SAW duplexer in which characteristics can be remarkably improved.
Additional objects, advantages and novel features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
According to a first aspect of the present invention, a surface acoustic wave duplexer includes a piezoelectric substrate; a transmitting filter formed on the piezoelectric substrate; a receiving filter formed on the piezoelectric substrate; a transmitting (Tx) branching line formed on the piezoelectric substrate; and a receiving (Rx) branching line formed on the piezoelectric substrate.
According to a second aspect of the present invention, a portable communication device includes an antenna, a power amplifier, and a surface acoustic wave duplexer. The surface acoustic wave duplexer includes (1) a piezoelectric substrate; (2) a transmitting filter which is formed on the piezoelectric substrate and is connected to the power amplifier; (3) a receiving filter which is formed on the piezoelectric substrate; (4) a transmitting (Tx) branching line which is formed on the piezoelectric substrate and is connected between the antenna and transmitting filter; and (5) a receiving (Rx) branching line which is formed on the piezoelectric substrate and is connected between the antenna and receiving filter.